1. Field of the Invention
The present invention relates to the optical signal intensity modulator using the electro-optic effect.
The optical intensity modulator is a required component for transmitting part or the signal processing part in the optical communication. In transmitting part, this modulator is usually used as an external modulator of a continuous oscillation light source and functions as transforming the electric signal into the optical signal. In signal processing part, the modulator usually acts as a switch for passing or cutting off the incident optical signal.
Most optical intensity modulators widely commercialized use the electro-optic effect of LiNO3. The electro-optic effect means that the index of refraction of medium is changed according to the degree of applied electric field, and the value thereof is the electro-optic coefficient.
The configuration and the principle of operation of optical intensity modulator using the electro-optic effect of LiNO3 are as follows. A waveguide like Mach-Zehnder interferometer is formed and electrodes capable of applying electric field to two optical paths or one optical path are formed, on the LiNO3 substrate. Therefore, this structure produces the phase difference between the two paths by the voltage applied from the outside when the light propagates the interferometer. The size of the phase difference causes the constructive interference or the destructive interference. The constructive interference makes the outputted optical intensity maximum and the destructive interference makes it minimum.
At this time, the voltage difference producing the constructive and destructive interferences is called a switching voltage and is referred to Vπ. Therefore, when the modulator is driven by the Vπ, the intensity of the light propagated through the inteferometer can be switched on and off by the maximum and minimum intensity. The switching voltage is inverse proportional to the multiplication of the length of optical path with voltage applied and the electro-optic coefficient of LiNO3. The higher the performance of the modulator is, the smaller the value of the switching voltage is. And the value of the commercial products is about 5V.
Meanwhile, to use the electro-optic coefficient in the most effective way, it is necessary to make the directions of electric field same as that of light polarization. Therefore, most commercialized LiNO3 optical modulators use polarization-maintained optical fibers for the input end thereof. In the case that the input light is linearly polarized and is exactly incident to the polarization axis of the polarization maintained optical fiber, the intensity of the input light is modualted in the most effective way. Otherwise, modulation may not be occurred in the worst case. In other words, LiNO3 optical intensity modulator commercialized and widely used has extreme sensitive property to the polarization state of input light.
Even if the modulator is sensitive to the polarization, it is possible to use the modulator in the transmitting part for producing the optical signal. Since a linearly polarized light is outputted from the semiconductor laser as a light source, the input signal can exactly arranged to the polarization axis of a polarization-maintained optical fiber on the input end of the modulator. But, when the modulator is used amid the transmission line, light polarization is irregularly changed while the light propagates, so that the characteristic independent of polarization becomes important. When the modulator sensitive to the polarization is used, polarization controller for adjusting the polarization of input light must be inserted before the input terminal. When the single wavelength is inputted to this modulator, potential problem can be relatively solved, but in the case that the WDM (wavelength division multiplexed) optical signal having various wavelengths is inputted, the situation becomes serious. As the wavelengths of each signal have different polarizations respectively, they can be operated in one wavelength and can not be operated in other wavelengths at all.
Therefore, in that case, the wavelengths must be separated per each wavelength using the WDM demuxer, and after the polarization of each wavelength signal is adjusted, the wavelengths with each of the adjusted wavelengths should be incident on the optical intensity modulator using the WDM muxer.
However, in the case that the optical intensity modulator is used for the optical signal processing element, the need for polarization-independent optical intensity modulator is extremely increasing, since it is difficult to integrate the polarization controller.
Meanwhile, there is another type of optical polymeric modulator as an optical modulator using the electro-optic effect. Even though the optical polymer has disadvantages of thermal instability and optical loss, it has been researched for its own characteristics of high speed modulation even more than 100 GHz, big electro-optic coefficient with about 100 pm/V, relatively easy manufacturing process, and integration capability, etc. In order to increase the electro-optic coefficient in the optical waveguide configured using the electro-optic polymer, the polymer must be poled.
2. Description of the Prior Art
Hereinafter, the electro-optic polymer of the prior art will be explained with reference to the FIGS. 1A and 1B.
FIG. 1A shows a plane view, and FIG. 1B shows the cross section of I-I′ in FIG. 1A. A Mach-Zehnder interferometer is not appeared on the surface covered by the upper cladding layer 16, marked as a dotted line in the plane view, and as squares 18a, 18b with oblique lines in the cross section. The electrodes 11, 17 are marked as rectangles with oblique lines in the plane view and as thick solid lines on the substrate and the upper cladding layer respectively. The arrow of thick solid lines means the polarization direction and the arrows of thin dotted lines mean the direction of electric field.
Meanwhile, when high voltage is applied with a temperature more than a predetermined temperature after the two electrodes are formed parallel with the optical waveguide therebetween, polymer molecules are arranged to the direction where the electric field is applied, and this defines the poling direction.
Referring to FIGS. 1A and 1B, a structure having an upper cladding layer 16, a core layer 14, a lower cladding layer 12, electrodes 11, 17, and a substrate 10 in that order is shown in the cross section.
In other words, the electrodes are formed at one optical path of the Mach-Zehnder interferometer, wherein a voltage (V) is applied to the electrode 11 and a ground voltage is applied to the electrode 17. At this time, the phase of the light propagating through the path (A) changes, thereby the phase of the light propagating through the path (B) without any electrodes differs from the phase of path (A), so that the phase difference occurs. Constructive or deconstructive interference occurs in accordance with the phase difference, and the voltage differences at which the constructive or deconstructive interference occurs correspond to the switching voltages. The switching voltage becomes smallest in the case that the polarization direction of the input light is same as the poling direction of electro-optical polymers, and biggest in the case that the poling direction of the input light is perpendicular to the polarization direction of electro-optical polymers. Therefore, modulation characteristics greatly change to the polarization state of the input light.
When a driving voltage is applied to the poled waveguide, the phase of the light being propagated changes due to the electro-optic effect. At this time, the value of phase becomes biggest in the case of linearly polarized light having direction same as the poling direction and smallest in the case of linearly polarized light vertical to the poling direction. As the size of phase modulated is varied in accordance with the polarization of the input light for the same operating condition, the electro-optic optical polymeric intensity modulator of Mach-Zehnder interferometer type depends on the polarization.
Meanwhile, Min-Cheol, OH et al discloses the structure of polarization-independent optical modulator that polarization converters for making the polarization vertical are formed amid each of the two optical paths of the M-Z interferometer, and the disclosure is entitled “Polymeric polarization-independent modulator incorporating . . . ” (Photonics Technology Letters, Vol. 8, No. 11, pp 1483-1485). However, the polarization adjusting devices are inserted amid the optical path of Mach-Zehnder interferometer, so that the structure becomes more complicated than the conventional modulator structure, and also produces excessive optical losses due to the electro-optical polymers extended by the polarization adjusting device.
Also, U.S. Pat. No. 5,751,867 to J. H. Schaffner et al, entitled “Polarization-insensitive electro-optic modulator”, describes the polarization-insensitive optical intensity modulator that the directions of the two optical paths of Mach-Zehnder interferometer is perpendicular each other.
Hereinafter, the polarization-insensitive electro-optical polymeric modulator of the prior art will be explained with reference to the FIGS. 2A and 2B.
As shown in the FIGS. 2A and 2B, all electrodes 21a, 21b and 21c are coplanar formed between the substrate 20 and the lower cladding layer 22. Therefore, when a voltage is applied after the electrodes are connected as shown in FIG. 2A, electric fields are formed in the directions marked as circular solid lines, thereby the two paths of the Mach-Zehnder interferometer become perpendicularly poled each other as shown in solid line.
Therefore, this method can not use the applied voltage in an effective way, because the intensity of the electric field formed along the shortest distance of the electrodes is bigger than that of the electric field circularly formed when a voltage is applied between the two electrodes. Referring to FIG. 2B, the electric field generated in the straight direction where the three electrodes are placed is strong, while that formed in the circular direction for perpendicularly poling the two optical paths each other is relatively weak. Therefore, the aforementioned structure can not use the applied voltage in an effective way, and the problem occurs like that the voltage necessary for switching on-off the optical intensity becomes bigger.
In other words, the polarization-insensitive optical intensity modulators of the prior art have no further elements in the Mach-Zehnder interferometer structure and only change the positions of poling and electrodes for driving a little., so that they have the advantages of having polarization-insensitive characteristics and relatively similar manufacturing processes like the usual modulator, while have the disadvantage that they can not use the applied voltages in an effective way since the electrodes for perpendicularly poling are coplanar types.